FIG. 1 is a structural diagram illustrating a Long Term Evolution (LTE) system which is a mobile communication system. The LTE system is an evolved version of a conventional UMTS system and has been standardized by the 3GPP (3rd Generation Partnership Project).
The LTE network may be generally classified into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and a Core Network (CN). The E-UTRAN includes at least one eNode-B serving as a base station and an Access Gateway (AG) located at the end of the network so that it is connected to an external network.
The AG may be classified into a portion for processing user traffic and a portion for processing control traffic. The AG portion for processing user traffic and the AG portion for processing control traffic may be connected to each other via a new interface for communication. One or more cells may exist in an eNode-B. The eNode-Bs may be connected by an interface for the transmission of user traffic or control traffic.
The CN includes the AG and a node for registering a user of the user equipment (UE). An interface may also be provided in the E-UMTS in order to classify the E-UTRAN and the CN.
Radio interface protocol layers may be classified into the first layer (L1), the second layer (L2), and the third layer (L3) on the basis of three lower layers of an Open System Interconnection (OSI) reference model that is well known in the art. A physical layer of the first layer (L1) provides an information transfer service over a physical channel. A radio resource control (RRC) layer located at the third layer (L3) controls radio resources between the UE and the network.
The RRC layer exchanges RRC messages between the UE and the network for this purpose. The RRC layer may be distributed to a plurality of network nodes, such as eNode-B and AG, and may also be located at the eNode-B or the AG.
FIG. 2 is a conceptual diagram illustrating a control plane of a radio interface protocol structure between the UE and the UTRAN (UMTS Terrestrial Radio Access Network) based on the 3GPP radio access network standard. The radio interface protocol is horizontally represented by a physical layer, a data link layer and a network layer. The radio interface protocol is vertically represented by a user plane for transmitting data and the control plane for transmitting control signals.
The protocol layers of FIG. 2 may be classified into a physical layer, a Medium Access Control (MAC) layer, a Radio Link Control (RLC) layer and a Radio Resource Control (RRC) layer.
The physical layer, which is a first layer, provides an information transfer service to an upper layer over a physical channel. The physical layer is connected to a Medium Access Control (MAC) layer located there above via a transport channel.
The MAC layer communicates with the physical layer over the transport channel such that data is communicated between the MAC layer and the physical layer. Data is communicated among different physical layers, such as between a first physical layer of a transmission side and a second physical layer of a reception side.
The MAC layer of the second layer (L2) transmits a variety of services to the RLC (Radio Link Control) layer, which is its upper layer, over a logical channel. The RLC layer of the second layer (L2) supports reliable data transmission.
It should be noted that the RLC layer is depicted in dotted lines, because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself may not need to exist.
The RRC (Radio Resource Control) layer located at the lowest portion of the third layer (L3) is defined by only the control plane. The RRC layer controls logical channels, transport channels and physical channels for the configuration, reconfiguration and release of Radio Bearers (RBs). An RB denotes a service provided by the second layer (L2) for data transfer between the UE and the E-UTRAN.
FIG. 3 is a conceptual diagram illustrating a user plane of a radio interface protocol structure between the UE and the UTRAN according to the 3GPP radio access network standard. The radio protocol user plane classifies into a physical layer, a MAC layer, a RLC layer and a PDCP (Packet Data Convergence Protocol) layer.
The physical layer of the first layer (L1) and the MAC and RLC layers of the second layer (L2) is used to effectively transmit data using an IP packet, such as IPv4 or IPv6, on a radio interface with a relatively narrow bandwidth. The PDCP layer performs header compression to reduce the size of a relatively-large IP packet header containing unnecessary control information.
Uplink and downlink channels for transmitting data between the network and the UE will hereinafter be described in detail. Downlink channels transmit data from the network to the UE. Uplink channels transmit data from the UE to the network.
Examples of downlink channels are a Broadcast Channel (BCH) for transmitting system information and a downlink Shared Channel (SCH) and a Shared Control Channel (SCCH) for transmitting user traffic or control messages. The use traffic or control messages of a downlink multicast service or broadcast service may be transmitted over the downlink shared channel (SCH) or may be transmitted over an additional multicast channel (MCH).
Examples of uplink channels are a Random Access Channel (RACH) and an uplink shared channel (SCH) and a shared control channel (SCCH) for transmitting user traffic or control messages.
FIG. 4 is a conceptual diagram illustrating a hybrid automatic repeat and request (HARQ) scheme. A method for implementing HARQ in the downlink physical layer of a radio packet communication system will be described with reference to FIG. 4.
Referring to FIG. 4, the eNode-B determines a UE that is to receive packets and the type of packet that is to be transmitted to the UE, such as a code rate, a modulation scheme and an amount of data. The eNode-B informs the UE of the determined information over a High-Speed Downlink Shared Control Channel (HS-SCCH) and transmits a corresponding data packet via High-Speed Downlink Shared Channel (HS-DSCH) at a time associated with the transmission of the information over the HS-SCCH.
The UE receives the downlink control channel, identifies a packet type to be transmitted and a transmission time point, and receives the corresponding packet. The UE then attempts to decode the received packet data.
The UE transmits a negative acknowledgement (NACK) signal to the eNode-B if the UE fails to decode a specific packet, such as data1. The eNode-B recognizes that packet transmission has failed and re-transmits the same data, such as data1, using the same packet format or a new packet format at a suitable time point. The UE combines the re-transmitted packet, such as data1, and a previously received packet for which packet decoding failed and re-attempts packet decoding.
The UE transmits an acknowledgement (ACK) signal to the eNode-B if the packet is received and decoded successfully. The eNode-B recognizes successful packet transmission and performs transmission of the next packet, such as data2.
A random access channel (RACH) indicates a channel for transmitting an initial control message from the UE to the network. The RACH is adapted to implement synchronization between the UE and the network. Furthermore, if there is no more data for transmission left in a UE that desires to transmit data in an uplink direction, the UE can acquire necessary radio resources over the RACH.
For example, when the UE is powered on it attempts to access a new cell. The UE performs downlink synchronization and receives system information from a target cell desired by the UE.
Upon receiving the system information, the UE must transmit an access request message to access the RRC layer. However, the UE is not synchronized with a current network and there is no guarantee of uplink radio resources since it uses the RACH.
In other words, the UE requests radio resources capable of transmitting the access request message to the network. If the eNode-B receives the radio-resource request signal from the UE, it allocates suitable radio resources to the UE to transmit a RRC connection request message. The UE can then transmit the RRC connection request message to the network using the allocated radio resources.
In another example, it is assumed that an RRC connection is established between the UE and the network. The UE receives radio resources from the network according to the radio resource scheduling process of the network such that data from the UE is transmitted to the network using the radio resources.
However, if there is no more data for transmission left in a buffer of the UE, the network no longer allocates uplink radio resources to the UE. If the network allocates the uplink radio resources to the UE, this allocation is considered to be ineffective. The buffer state of the UE, is periodically or accidentally reported to the network.
Therefore, if new data is stored in the buffer of the UE having no radio resources, the UE utilizes the RACH since there are no uplink radio resources allocated to the UE. In other words, the UE requests radio resources required for data transmission to the network.
RACH as used in a Wideband Code Division Multiple Access (WCDMA) system will hereinafter be described in detail. The RACH is used for transmission of data with short length. Some RRC messages, such as an RRC connection request message, a cell update message, and a URA update message, are transmitted over the RACH.
A plurality of logic channels can be mapped to the RACH. For example, a common control channel (CCCH), a dedicated control channel (DCCH), and a dedicated traffic channel (DTCH) may be mapped to the RACH. The RACH is mapped to a physical random access channel (PRACH).
FIG. 5 is a conceptual diagram illustrating an example of a PRACH (Physical Random Access Channel) transmission method. As illustrated in FIG. 5, the PRACH which is an uplink physical channel is divided into a preamble part and a message part.
The preamble part performs a power-ramping function for adjusting power required for transmitting a message and an anti-collision function for preventing transmissions from several UEs from colliding with each other. The message part performs transmission of a MAC Protocol Data Unit (MAC PDU) to the physical channel from the MAC layer.
If the MAC layer of the UE indicates the physical layer of the UE to transmit the PRACH transmission, the physical layer of the UE selects a single access slot and a single signature and transmits the PRACH preamble in the uplink. The preamble can be transmitted during an access slot period of 1.33 ms and selects a single signature from among 16 signatures during an initial predetermined period of the access slot such that it can transmit the selected signature.
When the UE transmits the preamble, the eNode-B can transmit a response signal over an acquisition indicator channel (AICH) which is a downlink physical channel. The eNode-B transmits a positive response (ACK) or negative response (NACK) to the UE using a response signal transmitted over the AICH.
If the UE receives an ACK response signal, it transmits the message part. If the UE receives a NACK response signal, the MAC layer of the UE indicates the physical layer of the UE to perform PRACH retransmission after a predetermined time. If the UE does not receive a response signal corresponding to the transmitted preamble, it transmits a new preamble at a power level that is higher than that of a previous preamble by one level after a designated access slot.
Although the above-mentioned description has disclosed a response signal to the RACH preamble, it should be noted that the eNode-B can transmit data or control signals to the UE. There are a variety of control signals transmitted from the eNode-B to the UE, such as downlink scheduling information, uplink scheduling grant information, and response information associated with the UE s RACH preamble transmission.